3GPP (3rd Generation Partnership Project) defines HNB (Home Node B) as a small base station that can be installed inside an office or a user's premises, to allow user equipment (UE) to gain access to an operator network via a HNB. A radio area covered by a single HNB is also referred to as small cell, micro cell, femtocell, picocell, or the like as it has a small range such as the inside of an office or a user's premises. Hereinafter, “femtocell” will be used for convenience, assuming that it includes these cells. In general, such femtocells are configured in large numbers within a macrocell, which covers a wide area.
3GPP defines PSC (Primary Scrambling Code) as a cell's physical cell identification information. Each cell uses a different PSC from one another, whereby a cell can be identified, and an association is made from the PSC to the cell's logical cell identification information (Cell Identity). However, the physical cell identification information is available in limited numbers. For example, a maximum of 512 PSCs are available in UMTS (Universal Mobile Telecommunications System), and a maximum of 504 Pas (Physical Cell Identities) are available in LTE (Long Term Evolution) System. Accordingly, a PSC is used multiple times for femtocells, which are configured in large numbers within a macrocell, and so a unique association cannot be made from a PSC to a Cell Identity, which causes PSC ambiguity. Specifically, in case of a legacy UE prior to 3GPP Release 9, that is, a UE that does not support a function (SI Acquisition function) of acquiring the system information of any cell other than a cell to which the UE is wirelessly connected, the RNC (Radio Network Controller) of a handover-source macro network cannot uniquely determine a target cell that corresponds to a PSC reported from this UE. NPLs 1 and 2 disclose methods (disambiguation) for solving such PSC ambiguity (=PSC confusion).
Hereinafter, methods for PSC disambiguation at a RNC and a HNBGW (HNB gateway) will be described, using a system shown in FIG. 1 as an example.
A) PSC Disambiguation at RNC
According to Annex C.2 (pp. 75-76) of NPL 1, for a first step, a RNC stores necessary information in a database during a process for handing over (handing out) a UE connected to the HNB of a femtocell from the femtocell to a macrocell. Subsequently, for a second step, the information in the database is used when the UE is handed in, whereby a target cell (the femtocell) is identified.
Specifically, in the first step, the UE located in the femtocell sends a Measurement Report to the HNB, and the HNB, when receiving it, sends a handover request message to the RNC via a HNBGW. The RNC stores information of the handover request message in the database and performs learning. This database information includes the logical cell identification information (Cell Identity) of the femtocell and the PSC thereof, the Cell Identity of the macrocell under the RNC's control and the PSC thereof, and the time difference between the reference times of the femtocell and the macrocell measured by the UE (Delta Observed Time Difference: Delta_OTD).
Subsequently, in the second step, the RNC, when receiving a Measurement Report from the UE located in the macrocell, acquires the Cell Identity of the femtocell that is a handover target from the database information and sends a handover request message to the HNB of this femtocell via the HNBGW. The Cell Identity of the femtocell is set in this handover request message, and this Cell Identity is the Cell Identity set in UE History Information in the handover request message sent during hand-out in the first step.
B) PSC Disambiguation at HNBGW
According to Annex C.3 (pp. 76-77) of NPL 1, a method is basically similar to the method for disambiguation at RNC according to C.2, with the difference that the location where the database is stored is the HNBGW.
First, in a first step, a handover request message is sent from the HNB as described above, but the HNBGW, upon receiving the handover request message, constructs database information based on the message and forwards the handover request message to the RNC. The database information includes the logical cell identification information (Cell Identity) of the femtocell and the PSC thereof, the Cell Identity of the macrocell under the RNC's control and the PSC thereof, and delta_OTD information concerning the femtocell and its neighboring macrocell. The delta_OTD represents the time difference between the reference times of the macrocell and the femtocell measured by the UE.
Subsequently, in a second step, the UE sends a Measurement Report to the RNC, and the RNC sends a handover request message including the Measurement Report to the HNBGW. The HNBGW selects the femtocell that is a handover target based on a Cell Identity and the Measurement Report set in UE History Information in the handover request message and on the constructed database information, and sends the handover request message to the HNB of this femtocell.